The present invention relates generally to crystal oscillators and more particularly to a circuit for detecting stable operation of a crystal oscillator.
Crystal oscillators are a vital part of many integrated circuit devices such as a system-on-chip (SOC). Crystal oscillators are used to provide an accurate and stable clock reference for the SOC and its associated phase-locked loops (PLL). Typically, crystal oscillators take some time to stabilize, with oscillations growing as an envelope over a certain time period until the amplitude of the oscillations reaches a stabilized level. The time that these oscillations take to stabilize depends on factors like load capacitance, aging, and ambient operating conditions.
Known crystal oscillator monitoring circuits observe the digital clock output from the crystal oscillator and measure its duty cycle but provide no information about the sinusoidal oscillations (i.e., the analog clock output) also produced by the crystal oscillator. Some SOC devices use a counter-based fixed delay (typically between 1 ms and 5 ms) after start-up before running the SOC. However, as a crystal oscillator's boot time is greatly dependent on parasitic components, a boot time of anything up to 100 ms is possible.
Therefore it would be beneficial to provide an indication that a crystal oscillator has reached a stable region of operation.